He gained notice for his involvement in finding the Lost Patrol, three NWMP officers and an ex-NWMP officer/guide who disappeared on the trail between Fort McPherson and Dawson City in the winter of 1910–11.

In 1907, he began participating in the annual mid-winter dog sled mail patrols between the NWMP detachments in Dawson City and Fort McPherson, a trip of 800 kilometres (500 mi) through the wilderness.

[3] The time required for the trip depended heavily on the weather; previous patrols had taken as few as 14 days and as many as 56 for the one-way journey.

[3] In late December, the patrol encountered local Kutchin families but Fitzgerald did not choose to hire one as a guide.

[4] When the patrol failed to arrive in Dawson City by late February 1911, Corporal Dempster was dispatched with Constables J.F.

[2] Dempster continued to lead the winter patrols for several years, establishing several speed records, including 14 days in 1920.

He was able to achieve his patrols safely, avoiding unnecessary risks and employing local First Nations guides when required.

[2] He eventually reached the rank of Inspector, retiring from the Royal Canadian Mounted Police (successor to the NWMP) in 1934.